FLT3 ligand is essential for sustaining the peripheral B-cell compartment in adult mice. Mean (SEM) values (from 3-8 mice of each genotype and age) of CLPs (A; Lin⁻SCA-1^loKIT^loIL-7Rα⁺), pre-pro-B (B; B220⁺CD43⁺AA4.1⁺CD19⁻), pro-B (C; B220⁺CD43⁺AA4.1⁺CD19⁺), pre-B (D; B220⁺CD43⁻IgM⁻), and mature/immature B cells (E: B220⁺IgM⁺) in BM, and of B cells (F; B220⁺IgM⁺) in PB in 2-week-old and 27-week-old WT and FL^−/− mice. (G) Mean (SEM) values (from 5 fetuses per genotype) of pre-pro-B (B220⁺CD43⁺AA4.1⁺CD19⁻) and pro-B (B220⁺CD43⁺AA4.1⁺CD19⁺) cells in day 14.5 fetal liver from WT and FL^−/− mice.